Voltage and current reference circuits find many applications in electronic circuits including Flash and other types of electronic memory device applications. The bandgap reference circuit is a common circuit solution for supplying a voltage or current reference for such applications. FIG. 1 is a prior art bandgap circuit 100 and operates generally as follows. P1 and P2 act as a standard MOS current mirror providing current to Q1 and Q2, which are configured as a bipolar current mirror. Q1 and Q2 are sized differently; therefore, although they conduct the same current, they have different current densities. Therefore, there will be a difference in their Vbe voltages and the difference will be reflected in the current through R1. VREF is a voltage reference that is a function of the current through R2 and the base-emitter voltage Vbe of Q3. Since the current through R2 is mirrored from P1 it is seen that the current through P3 is a function of ΔVbe between Q1 and Q2 and R1. Therefore, VREF is a function of the ΔVbe between Q1 and Q2, the ratio in resistor values R1 and R2, and Vbe of Q3. The current mirror insures equal collector currents IC an saturation currents IS through Q1 and Q2. Note that Q1 is n times bigger than Q2, thus:ΔVbe=VBE,Q2−VBE,Q1=VTln(IC/IS)−VTln(IC/nIS)=k(T/q)ln(n).ΔVbe exhibits a positive temperature coefficient (+TC). If the positive temperature coefficient of ΔVbe is combined with VBE,Q3, which has a negative temperature coefficient (−TC), along with the correct weighting ratios of R1 and R2, VREF will have approximately a zero temperature coefficient, and VREF will be independent of temperature. This ratio is determined by taking the equation for VREF that incorporates all temperature dependencies, differentiating with respect to temperature, and setting the equation equal to zero. For example, from FIG. 1, we can calculate VREF as:VREF=VBE,Q3+R2(mIC)=VBE,Q3+R2(m ΔVbe/R1)=VBE,Q3+m(R2/R1)ln(n)kT/q and:  (1)∂VREF/∂T=∂Vbe/∂T+m(R2/R1)ln(n)k/q   (2)As discussed, to have a reference that is substantially independent of temperature, equation (2) should be zero, or:∂VREF/∂T=∂Vbe/∂T+m(R2/R1)ln(n)k/q=0   (2)′If we assume a typical value of positive temperature coefficient for ∂Vbe/∂T:∂Vbe/∂T=−1.5 mV/°KWhen this value is substituted into equation 2′, and solved for VREF, a new value for VREF is obtained having a zero temperature coefficient, where:VREF=1.25VThis is well known by those skilled in the art of bandgap reference circuits.
The above explanation of prior art circuit 100 of FIG. 1 assumes that the gain-bandwidth product of the reference circuit. temperature, operation speeds, and manufacturing tolerances remain within limited bounds. However, in many cases, this is not a valid assumption. Often, integrated circuits must operate, for example, combinations of high speeds, extreme temperatures, extreme process corners, and low voltages. Under some of these conditions, the gain-bandwidth product of the reference circuit may be inadequate.
Additionally, as device densities and speed requirements continue to increase, the speed requirement of the reference circuit may need to increase to keep pace with the remainder of the circuit, including a reference circuit used to supply, for example, the reference voltage for a word line or a voltage booster of a memory circuit. Further, as supply voltage levels decrease due to these higher density architectures, device speed requirements may be increasingly difficult to obtain, particularly at low supply voltage and reference levels, and at low operating currents over wide operating temperatures. These issues are particularly evident during read operations at low power supply voltages (Vcc's) wherein the read margin decreases, resulting in an inaccurate read at low supply voltages. In Flash devices, typically, the smaller the read margin at low Vcc's may be due to a reduced reference voltage at low Vcc.
It should also be noted that in the typical bandgap reference circuit of FIG. 1, the current mirror is usually in the cascode form to reduce the variation of VREF with respect to the supply voltage VCC. The particular arrangement of bandgap voltage reference of FIG. 1, however, can not be used directly for the high speed circuits being considered, because of reduction in the gain-bandwidth product of the reference at higher speeds and low power supply voltages. Accordingly, there is a need to provide a means of compensation that reduces the negative effects of a low VCC supply voltage applied to a reference voltage circuit operating at high speeds and low power supply and reference levels, while accommodating a wide range of temperature and process variations.